In the production of J-traps which are molded of a rigid plastics material, it is desirable for the U-shaped channel or passage to be of substantially uniform circular cross-section with minimum internal seams and without internal flash. Thus it is desirable to use a one-piece U-shaped core element instead of sectional core elements or members such as those disclosed, for example, in U.S. Pat. No. 454,186 for molding lead traps. A one-piece U-shaped core element may be used in the molding of a flexible J-trap or tube, for example, as disclosed in U.S. Pat. No. 3,028,630, since the flexible trap may be deformed when it is removed from the core element. However, a one-piece U-shaped core element is difficult or awkward to remove from a molded J-trap or tubular conduit when the conduit is molded of a rigid plastics material.
It is also desirable to eliminate all manual operations involved in inserting and removing core sections or elements during the molding of a rigid plastics J-trap or conduit so that the molding operation may be completely automatic and the cost of the conduit is minimized. In the art of injection molding L-shaped conduits or elbows, multiple cavity molds have been constructed or proposed for automatically producing the elbows without any manual operations. For example, U.S. Pat. Nos. 3,632,277 and 3,746,493 disclose multiple cavity molds for automatically producing elbows of rigid plastics material. However, the molds disclosed in these patents are not adaptable for molding U-shaped or "return" conduits such as J-traps or other conduits having a "return bend."